1. Field of the Invention
The present invention relates to an austenitic stainless steel excellent in blankability, especially fine blankability.
2. Description of Related Art
Shear process, especially blanking, with a press has been applied to various kinds of metal sheets such as common steel, stainless steel and nonferrous metal, since the metal sheets can be efficiently sized to an objective shape. However, a plane formed by blanking is rugged with poor dimensional accuracy; a metal sheet is likely to be drooped at its broader surface, and thickness of the metal sheet is reduced at a part near the blanking plane.
A blanking plane, which is generated by blanking a metal sheet, comprises a shear plane and a fracture plane. The shear plane has a smooth surface, while the fracture plane worsens dimensional accuracy of a blanked product. A shear plane ratio is calculated by dividing a surface area of the shear plane by a total surface area of the shear and fracture planes.
When blanking is adopted to a process for manufacturing a product which needs high dimensional accuracy, a blanking plane is ground by post-treatment such as barrel finishing. Such post-treatment is basically an extra process and causes poor productivity. In this regard, a fine blanking method has been adopted for manufacturing a product with high dimensional accuracy. In the fine blanking method, clearance is determined at a very small value to suppress formation of a fracture plane, and inflow of metal is suppressed to reduce generation of drooping during blanking.
On the other hand, stainless steel has been used so far for use exposed to a corrosive or high-temperature atmosphere. Especially, SUS 304 is representative stainless steel suitable for such use.
SUS 304 austenitic stainless steel is a hard material, causing the life of fine blanking dies to be shortened. The hardness of SUS 304 austenitic stainless steel also causes an increase of a ratio of a fracture plane, which degrades quality of a blanking plane, as well as increase of drooping. Even if a shear plane is formed with high dimensional accuracy by blanking, a working cost is higher compared with a cost for blanking common steel. Accounting these disadvantages, SUS 304 austenitic stainless steel is blanked by a usual method and then ground for manufacturing a product which shall have a blanking plane with high dimensional accuracy.